The invention relates to a method and apparatus for effecting prehydrolysis of comminuted cellulosic fibrous materials (such as wood chips, straw, bagasse, agricultrual wastes, etc.), and for the treatment of such material to effect hydrolysis and subsequent kraft pulping thereof.
Prehydrolysis is practiced to effect removal of hemi-cellulose, acid soluble lignins, and sugars such as glucose, galactose, mannose, arabinose, and zylose. By practicing prehydrolysis according to the invention, the majority of the hemi-cellulose is hydrolyzed to monoses, and the remaining hemi-cellulose is degraded sufficiently to make it more easily removed during the kraft cook. Little cellulose is hydrolyzed during prehydrolysis, although some acid soluble lignins--as mentioned above--are dissolved. Kraft cooking of the prehydrolyzed cellulosic fiber material performs the primary delignification to produce high alpha cellulose pulp. Prehydrolysis is necessary for the production of dissolving pulp, or for use in the viscous or acetate processes, and in order to facilitate production of a pulp with a 94-97% alpha cellulose content from a kraft digester.
In the prior art there have been two basic approaches to acid prehydrolysis.
In the first prior art approach, a single vessel is utilized to effect prehydrolysis and digestion. Such practice is exemplified by U.S. Pat. Nos. 3,380,883 and 3,413,189. While such an approach can achieve some desired results, it does not allow the separate recovery of the hydrolysate, which is increasingly valuable. Also when scaling or "gunking" occurs due to the mixing of the hydrolysate and kraft liquors, the system must be shut down and cleaned.
The second prior art approach is exemplified by U.S. Pat. No. 4,174,997 and a paper entitled "Development of the Alva Prehydrolysis Process" by Blom, et al, TAPPI 1981 Pulping Conference, pages 409-416. Two-vessel systems have the advantage of maintaining a cleaner separation between the acidic liquors in the prehydrolysis system and the caustic liquors in the kraft system, and provide a clear temperature separation between the lower temperature (e.g., 240.degree.-245.degree. F.) of the prehydrolysis zone and the higher temperature (e.g., 290.degree.-350.degree. F.) of the kraft zone. Typically, hydrolyzing liquid and the material are passed together in a co-current manner, and hydrolyzed compounds are removed from the material through the combined processes of displacement and diffusion. While such procedures can be effective, there is a tendency toward process instability due to variations in the moisture content and the bulk density of the incoming cellulosic material, which will directly affect uniformity of both the withdrawn hydrolysate and the pulp produced by the process.
According to the present invention, a method and apparatus are provided which substantially overcomes the above-mentioned problems. In the practice of the present invention, a two-vessel system is utilized in such manner that it is possible to minimize process variations. This is accomplished, in large part, by operating the hydrolysis reaction zone so that it is totally countercurrent. The practice of the present invention also increases the efficiency of the washing of the hydrolyzed compounds and the residual acids from the cellulosic material so as to reduce the transfer of these compounds to the kraft digester where they consume active chemicals and become involved in undesirable side reactions. The present invention also promotes the early removal of gases generated in the hydrolysis reaction, and thereby facilitates smooth movement of the cellulosic material downwardly through the treatment vessels. The present invention also improves the reaction driving force at the end of the hydrolysis reaction by providing the highest level of acidity (lowest pH level) at the point where the remaining unreacted hemi-celluloses are the lowest, thus the acid charge and/or temperature level of the hydrolyzing liquid can be lower than would be required by the prior art.
As used in the present specification and claims, "acid hydrolysis" means hydrolysis accomplished utilizing a wide range of temperatures and acidity of hydrolyzing liquids. That is, hydrolysis can be carried out at lower temperatures with concentrated acids, at intermediate temperatures with weak acid solutions, or at high temperatures with only water (acid being leached out of the material and contributing to the hydrolysis).
According to one aspect of the present invention, a method of prehydrolyzing comminuted cellulosic fibrous material (e.g., prior to kraft cooking thereof), utilizing a generally vertical vessel, is provided. The method comprises the steps of:
(a) Feeding comminuted cellulosic fibrous material suspended in liquid to a top portion of the vessel.
(b) Effecting countercurrent acid hydrolysis in a top zone of the vessel, as the material passes downwardly in the vessel.
(c) Withdrawing liquid containing recoverable hydrolysate, including hemi-cellulose and sugars, from the top of the acid hydrolysis zone of the vessel, and recirculating the liquid to be fed back to the top of the vessel in step (a).
(d) Diverting a part of the liquid, containing hydrolysate, withdrawn in step (c). And
(e) flashing the diverted liquid from step (d) to produce steam and a liquid with concentrated hydrolysate.
The method also contemplates effecting countercurrent washing of the hydrolyzed material in a portion of the vessel below the acid hydrolysis zone thereof, and controlling the upflow of wash liquid by controlling the proportion of liquid diverted in step (d). Typically the total time for passage of a particular unit of material through the acid hydrolysis reaction zone will be a predetermined time between about 30-180 minutes, the predetermined time set depending upon the particular cellulosic material being treated, and other reaction conditions.
According to another aspect of the present invention, there is provided a method for treating comminuted cellulosic fibrous material utilizing first and second generally vertical vessels. Such a method comprises the steps of continuously:
(a) Steaming the comminuted cellulosic fibrous material.
(b) Entraining the steamed cellulosic fibrous material in liquid, to form a suspension.
(c) Transporting the suspension to the top of the first vessel, in a first circulation loop.
(d) Separating liquid from the cellulosic fibrous material at the top of the first vessel, the separate liquid being transported in the first circulation loop, and including hydrolysate including hemi-cellulose, and sugars.
(e) Separating at least a part of the hydrolysate from the liquor in the first circulation loop.
(f) Passing the material downwardly in the first vessel.
(g) Subjecting the downwardly passing material to countercurrent hydrolyzing liquid in a top, hydrolysis reaction zone, of the first vessel.
(h) Effecting washing of the material at the bottom of the first vessel, below the hydrolysis reaction zone.
(i) Feeding the washed, prehydrolyzed, material from the bottom of the first vessel to the top of the second vessel in a second circulation loop. And
(j) effecting kraft digestion of the material in the second vessel.
Apparatus for practicing the exemplary cellulosic fibrous material treating method according to the present invention typically includes the following components: (a) A steaming vessel. (b) A continuous high pressure feeder for feeding steamed material, entrained in liquid, at high pressure. (c) A first generally vertical vessel. (d) A liquid-solids separator disposed adjacent the top of the first vessel. (e) A first circulatory loop between the high pressure feeder and the liquid-solid separator. (f) first withdrawal screens at a central vertical portion of said first vessel. (g) Hydrolyzing liquid introduction means adjacent the first withdrawal screens. (h) A hydrolyzing liquid circulatory loop, including a pump and heating means, operatively connected between the structures (f) and (g). (i) Means for introducing wash liquid at a bottom portion of said vessel, below the first withdrawal screens. (j) Means for discharging hydrolyzed material from the bottom of the first vessel. (k) A second generally vertical vessel, comprising a kraft continuous digester. (l) A second circulatory loop interconnecting the means (j) and the top of the continous kraft digester. (m) Means for diverting a portion of the liquid in the return portion of the first circulatory loop. And (n) means for removing hydrolysate from the liquid diverted by means (m).
It is the primary object of the present invention to provide a method and apparatus for treating cellulosic fibrous material to effect acid prehydrolysis thereof in an efficient and effective manner so as to produce pulp of uniform quality, maximize the efficiency of the subsequent kraft digestion stage, and effect withdrawal of high quality hydrolysate. This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.